The right behavior here is to signal the error. CLHS (section 6.1.1.7) says that "An error of type program-error is signaled (at macro expansion time) if the same variable is bound twice in any variable-binding clause of a single loop expression. Such variables include local variables, iteration control variables, and variables found by destructuring."

There is a simple patch. Before declaring a variable, we check whether its name is already used within the loop. To do that, we analyze *loop-declarations* list contents. Variables which are initialized by destructuring are described by the sublists in that list. Every sublist has a variable name as a last element.